It is conventional to use Hall-effect devices to measure the current in each of the legs of a three-phase load supplied by a pulse-width modulated inverter. Such inverters typically use three pairs of field-effect transistors (FETs) or equivalent to supply the three-phase load (which may be, for example, a three-phase AC motor). Hall-effect devices are large and expensive. It is an object of this invention to replace such devices with a small and economical solid-state circuit that makes use of sensing resistors in the return legs of the inverter circuit in combination with an estimator for receiving certain signals representing selected parameters and computing an estimated current value for each leg.
It is known to introduce a resistor into one or more of the return legs of the FETs of an inverter. See, for example, U.S. Pat. No. 5,825,641 issued to Mangtani on Oct. 20, 1998. However, Mangtani does not provide any provision to utilize the voltages developed across the sensing resistors to generate an estimate of the respective currents flowing in the legs of the load.
Voltages across sensing resistors have also been used to control brushless motors; see for example U.S. Pat. No. 5,469,033 issued to Huang on Nov. 21, 1995. However, Huang makes no provision for producing a signal representative of the current flowing in each leg of the load.